it my be counter-intuitive, but making it easy for a customer to end their subscription may be the best method of growing your business...(it will clearly have a dramatic impact on reducing your chargebacks!)
Here are a few thoughts;
1) have a link clearly visible on your home page for Customer Service
2) make it easy for a customer to cancel their subscription, albeit nothing wrong with offering alternatives to cancelling but nonetheless, just deal with it if that is what they want
3) if the cancel requires human intervention, have those people available 7 days a week, if not 7x24, but preferably have it be self-service
4) process refunds promptly
5) communicate clearly and repeat the message that the credit will take 7-21 days to show up online and up to 2 billing cycles to show up in paper statements
Why do these things make sense?
1) in this connected world we live in, word of mouth references are profoundly important and someone saying, "I used X and it was great and when I was finished with their service and went to cancel it was a piece of cake..." is much more likely to win you the next customer and potentially even this one back later when their needs change
2) it will clearly save you money in operations cost, chargeback fees, etc.
3) you will sleep better at night and be doing your part to give the subscription based business world a better name.